The present invention relates to radio transmitters and receivers. The object of the invention is to present a new method for producing a low frequency output signal from a frequency hopping input signal and a second method for producing a frequency hopping output signal from a low frequency input signal.
In particular, the invention relates to a method for producing, in a receiver, a first output signal, which has a specific first output frequency, from a first input signal the frequency of which changes quickly in steps of the size of the channel spacing or a multiple of the channel spacing of a mobile communication system within the frequency range of the mobile communication system. Furthermore, the invention relates particularly to a method for producing, in a transmitter, an output signal the frequency of which belongs to the frequency band of a mobile communication system, from a second input signal, which has a specific second input frequency and, further, for changing the second output frequency in steps of the size of the channel spacing or a multiple of the channel spacing of the mobile communication system, on the mobile communication system frequency band.
Further, the invention relates particularly to a receiver, a transmitter and a frequency synthesiser, wherein the method according to the invention is applied to a frequency synthesis, which implement the methods according to the invention. Methods according to the invention, the receivers and transmitters implementing the methods, as well as the frequency synthesiser based on the methods, can be used, in addition to the GSM system, in any radio system where the transmitting and receiving frequency is being changed.
In transmitters and receivers used in mobile communication systems, it must be possible to quickly change the transmitting and receiving frequency. This is particularly important in cellular networks that use frequency hopping. Frequency hopping is an efficient countermeasure for preventing harmful phenomena due to the radio channel""s properties and interference.
Rayleigh fade is a major problem especially in data transmission from a base transceiver station to a terminal. This can be seen particularly as a large variation in the received signal level. The variation in the level of the received signal is a problem that occurs particularly in slowly moving terminals. This means that a terminal may stay for a long time in a place where the received signal level is poor.
Rayleigh fade is frequency-dependent by its nature. In frequency hopping, successive bursts are sent at different frequencies. If the difference between these frequencies is sufficient, the fading properties of the frequencies in a radio channel can be considered to be independent of each other. In this case, the probability that two erroneous bursts are received decreases.
A second benefit to be gained by frequency hopping is interference diversity. In a busy traffic area, for example, in big cities, a network""s capacity is limited due to interference resulting from the re-use of frequencies. When the same frequency is being used in adjacent cells simultaneously, the frequencies interfere with each other. When using slow frequency hopping, the probability that the received signal and the interfering signal are simultaneously at the same frequency is reduced.
Adjustable local oscillators used in radio transmitters and receivers are often frequency synthesisers. A frequency synthesiser is normally based on a phase-locked loop the operation of which is based on an oscillator locked in a reference frequency.
A frequency synthesiser has a certain frequency resolution and settling time. The frequency of a signal produced by a frequency synthesiser can be changed in steps of the size of the frequency resolution or a multiplier of the frequency resolution. The changing of the frequency and locking in the desired frequency take place during the settling time. The settling time can be shortened by broadening the bandwidth of the filter of a phase-locked loop or by increasing the phase-comparison frequency. A disadvantage is that the increasing of the bandwidth of the phase-locked loop inevitably follows an increase in phase error. The phase-comparison frequency of conventional frequency synthesisers is equal to the frequency resolution. A fractional-n synthesiser has improved the settling time of conventional frequency synthesisers. The synthesiser in question is based on a conventional solution the phase-comparison frequency of which has been increased n-fold. Currently, mainly 5- and 8-fold fractional-n synthesisers are being used. For example, a basic solution, the frequency resolution and phase-comparison frequency of which are 200 kHz, can be speeded up by using, for example, a fractional-n synthesiser the frequency resolution of which is 200 kHz and the phase-comparison frequency 1 MHz (modulo-5).
There are several different means of implementation for the change of receiving and transmitting frequencies. In the basic solution of a Superheterodyne receiver, a local oscillator is used in a first intermediate frequency stage, the frequency of a local oscillator signal produced by it being adjustable. A frequency synthesiser, the frequency resolution of which is equal to the frequency resolution of the received signal, is used as the local oscillator. An input signal is down-converted into an intermediate frequency signal by mixing the intermediate frequency signal and the local oscillator signal. The intermediate frequency signal is further down-converted into an output signal by mixing the intermediate frequency signal and the local oscillator signal of a second intermediate frequency phase. The signal is detected from the output frequency. The first intermediate frequency is constant and the local oscillator of the second intermediate frequency stage operates at a constant frequency.
In the basic solution of a Superheterodyne transmitter, a local oscillator is used in a first intermediate frequency stage, the frequency of a local oscillator signal produced by it being constant. An input signal is up-converted by a first local oscillator signal into an intermediate frequency, the frequency of which is also constant. The intermediate frequency is further mixed by the local oscillator signal of a second intermediate frequency stage to a desired transmitting frequency. A transmit signal with the desired frequency is produced by adjusting the frequency of the second local oscillator signal and by mixing the second local oscillator signal and the intermediate frequency signal. The frequency of the local oscillator signal is the difference or sum of the transmit signal frequency and the intermediate frequency.
The problem with the basic solutions is the length of the settling time with the required frequency resolutions. For instance, in the GSM system (GSM, Global System for Mobile communications), channel spacing is 200 kHz and so the settling time of conventional frequency synthesisers is far too slow. On the other hand, the phase-comparison frequency of the currently available fractional-n synthesisers is normally 5- or 8-fold compared to the frequency resolution, and frequency locking takes about 40 phase-comparison periods of a phase-locked loop. This being the case, a phase-comparison frequency of 1.6 MHz (modulo 8) and 25 microsecond (=1/1.6 MHz*40 periods) settling times at best are achieved by fractional-n synthesisers, the frequency resolution of which is 200 kHz.
In a GSM network that uses frequency hopping, the frequency is changed after each burst. In between the bursts, there is a guard period, which is 8.25 bits long, i.e., which lasts for 30 microseconds. For example, in a base transceiver station, the changing and locking of a frequency should take place during a period less than half a guard period of a burst. Thus, the base transceiver station""s settling time should be distinctly less than 15 microseconds. In this case, the phase-comparison period can be 0.375 microseconds (=15 xcexcs/40 periods) at a maximum, i.e. the phase-comparison frequency must be no less than 2.67 MHz. This can be achieved by neither conventional frequency synthesisers, nor by even the fastest fractional-n synthesisers.
The settling time of the basic solution of a receiver described above can be speeded up by using, in a first intermediate frequency stage, two local oscillators according to FIG. 1. In the receiver shown in FIG. 1, in the first intermediate frequency stage, a switch, a mixer and two local oscillators are used by which a received signal is converted into an intermediate frequency signal. The local oscillators are connected to the switch, which has been connected to the mixer""s local oscillator input. A signal of the desired local oscillator is selected by controlling the switch.
The frequency of the local oscillators is adjustable. During a first timeslot, the signal produced by the first local oscillator is ready selected at the mixer input and the frequency of the signal of the second local oscillator is adjusted as desired. When the time slot changes to the next one, a second oscillator signal is switched to the mixer""s local oscillator input and the adjustment of the frequency of the first local oscillator signal is commenced.
In a second intermediate frequency stage, a single local oscillator is used. The intermediate frequency signal is converted in the mixer further into an output signal. This takes place by mixing the intermediate frequency signal with a signal produced by a third local oscillator. The third local oscillator operates at a fixed frequency.
An advantage of the solution described above is the speeding up of the time used for changing the frequency compared to the basic solution, because the frequency of one local oscillator can be adjusted at the same time as the other is producing the local oscillator signal to be used in the first intermediate frequency phase. The problem is that a local oscillator signal that has not been selected, leaks through the switch to the mixer""s local oscillator frequency input and produces harmful mixing products for the intermediate frequency. Furthermore, the signal leaking through the switch and the signal of the selected local oscillator produce intermodulation products that further produce harmful mixing products for the intermediate frequency. These harmful mixing products cannot be filtered off, because their frequencies are on the pass band of the intermediate frequency filter. The problem can be mitigated by using several series-connected switches, mechanical partitions or buffer amplifiers. However, the number of components required in the circuit increases, whereupon the circuit consumes more power and requires a larger surface area.
In one transmitter according to FIG. 2, a single local oscillator and mixer are used in a first intermediate frequency phase for converting an input signal into an intermediate frequency signal. This takes place by mixing the local oscillator signal of the first intermediate frequency phase with the intermediate frequency signal. The local oscillator in question operates at a fixed frequency.
In a second intermediate frequency stage, a switch, a mixer and two local oscillators are used for converting an intermediate frequency signal into an output signal. The local oscillators are connected to the switch, which is further connected to the mixer""s local oscillator input. A signal of the desired local oscillator is selected with the switch.
The frequency of the second intermediate frequency phase is adjustable. During a first time interval, the signal produced by the second local oscillator is ready selected at the local oscillator input of the mixer and the frequency of a signal produced by the third local oscillator is adjusted as desired. As the time interval changes to the next one, the third local oscillator signal is switched to the mixer""s local oscillator input and the adjustment of the second local oscillator signal is commenced.
An advantage of the transmitter according to FIG. 2 is the speeding up of the time required for changing the frequency according to the receiver described above. The problem is that harmful mixing products are produced and the large amount of components, the circuit""s power consumption and the need for surface area, as in the receiver solution shown in FIG. 1.
FIG. 3 shows a frequency synthesiser according to prior art. An attempt has been made to speed up the settling time of the frequency synthesiser by using two phase-locked loops instead of one. As in the solutions according to FIGS. 1 and 2, the frequency of the second phase-locked loop is adjusted at the same time as the first phase-locked loop is producing the frequency synthesiser output signal.
When transferring onto the next output frequency, the position of the switch is changed, whereupon the second phase-locked loop connects to the frequency synthesiser output and the adjustment of the frequency of the first phase-locked loop is commenced. The disadvantage of the solution in question is the large amount of components, because the switches require control and buffing against changing impedance. Therefore, the solution in question consumes a lot of power and requires a large surface area.
In the following, a new method will be presented for producing in a receiver an output signal, which has a specific frequency, from an input signal, the frequency of which changes in steps of the size of an input frequency resolution or a multiple of the input frequency resolution, on an input frequency band. In a method according to the invention, the input signal has an input frequency, which changes in steps of the size of the input frequency resolution or a multiple of the input resolution, on the input frequency band. The input frequency band is preferably within the frequency range of a GSM network and the input frequency resolution is preferably the difference between the GSM system channels. The output signal is preferably a signal, which is input to the receiver demodulator.
The basic idea of the invention is that when converting an input signal into an intermediate frequency signal, a value is set for the intermediate frequency from an intermediate frequency band, which comprises at least two different values. The difference between the values of the intermediate frequency band, i.e. the intermediate frequency resolution, is higher than the input frequency resolution. This, in turn, enables the use of local oscillators, the frequency resolution of which is higher than the input frequency resolution, in a receiver.
Normally, an input signal is converted into an intermediate frequency signal by down-converting, whereupon the intermediate frequency is lower than the input frequency. In some cases, the input signal can also be up-converted, whereupon the intermediate frequency is higher than the input frequency. Methods according to the invention operate well in either way. The intermediate frequency signal is further down-converted into an output signal, whereupon the output signal frequency is lower than the input signal frequency.
Further, a new method is presented for producing an output signal from an input signal, which has a specific input frequency, and for quickly converting the frequency of the output signal, on the output signal frequency band.
In the method, the output signal has an output frequency, which is changed in steps of the size of an output frequency resolution or a multiple of the output frequency resolution, on the output frequency band. The output frequency is preferably within the GSM system frequency range and the output frequency resolution is preferably the difference between the GSM system channels. The basic idea of the method according to the invention is the same as in the receiver, i.e. the conversion of an input signal into an intermediate frequency signal so that the intermediate frequency is set from at least two values belonging to an intermediate frequency band. The difference between the values that belong to the intermediate frequency band, i.e. the intermediate frequency resolution, is higher than the output frequency resolution.
In methods according to the invention, the conversion of input signals into an intermediate frequency signal and the down- and up-conversion of the intermediate frequency signal to output signals take place by mixing in a manner know to a person skilled in the art. The information contained by the input signal is preserved when converting the signal into an output signal.
Considerable advantages are gained by the inventions compared to solutions according to prior art. The gained advantages depend on whether conventional frequency synthesisers or fractional-n synthesiser are used in the solutions. In both cases, quick settling times are achieved by a transmitter and a receiver that implement the method, without impairing the frequency resolution. When comparing, for example, a solution according to the invention, where the intermediate frequency is selected from two frequencies, to a basic solution according to prior art, where conventional frequency synthesisers are used, the settling time of the solution according to the invention is nearly half of the solution according to prior art. In a solution according to the invention, the use of two intermediate frequencies enables the use of a local oscillator, the frequency resolution of which is twice the input frequency resolution and, thus, the division of the settling time in half compared to the basic solution of a transmitter. Correspondingly, three intermediate frequencies enable a three-fold frequency resolution of a first local oscillator compared to the input frequency resolution and, thus, the reduction of the receivers settling time to one third compared to a conventional solution. If conventional frequency synthesisers are replaced by fractional-n synthesisers, the receiver""s settling time decreases further.
When comparing a transmitter and a receiver that implement the methods according to the invention, to solutions where two local oscillators selected by a switch are used, the settling time of the transmitter or receiver according to the invention remains good, but the need for components and the power consumption of the circuit decreases and the surface area required by the circuit becomes substantially smaller. The decrease in power consumption and the need for smaller surface area result from the fact that no switch, switch control, buffer amplifiers or mechanical partitions are required. Neither do spurious responses and harmonic frequencies cause a problem, because only one local oscillator is used instead of two local oscillators selected with a switch.
According to the invention, it is also possible to implement a frequency synthesiser, which is considerably faster than a frequency synthesiser according to prior art and the frequency resolution of which is, however, at least as good as in a solution according to prior art. The advantages of a method according to the invention and of a frequency synthesiser based on said method are the same advantages as in the transmitter and receiver, i.e. a quick settling time and a simple implementation, because no switch, switch control, buffer amplifiers or mechanical partitions are required. Thus, the power consumption of the synthesiser decreases compared to a solution according to prior art, and the circuit consumes less power. Furthermore, the local oscillator frequency bands can be selected so that the mixer produces an output signal on the desired frequency band and the other mixing products are produced at a sufficient distance from the output signal frequency band. Therefore, after the mixer, no filtering is necessarily required, but the output signal obtained from the mixer can be directly switched, for example, to the next mixer.
The problems presented above can be avoided by methods according to the invention and by transmitters, receivers and frequency synthesisers implementing the methods. The settling times of transmitters, receivers and frequency synthesisers implemented according to the invention are reduced considerably without impairing the frequency resolution. The need for components in the circuits also decreases, whereupon the circuits consume less power and need a smaller surface area compared to solutions according to prior art. The time for changing the frequency of receivers and transmitters implemented according to the methods can be speeded up further independent of the time required for changing the frequency of the frequency synthesisers available each time. In addition to the advantages mentioned above, the advantages of transmitters, receivers and frequency synthesisers according to the inventions are also their ease of implementation and simple structure.
In particular, the inventions will benefit mobile stations where fast frequency settling time is needed in a base transceiver station, and mobile stations where fast frequency settling time, low power consumption and small circuits are required. The invention is characterised by what has been stated in the claims.